Re: NST filter failure?

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Mark,

At 07:18 PM 8/21/00 -0500, you wrote:
>Thanks for the info, Terry!  I concatenated the two posts and responded
>below.
>
>
>>         If the MOVs burned up (they short thank goodness!) they were
clipping
>> about 27kV spikes.  For them to get that hot, something must have been
>> seriously going overvoltage.  They did their job in saving the NST (the
>> hard way).  YEAH!!
>
>27kV PER LEG?!?!  I hope you mean total!  I'm not completely sure, but
>I'm pretty certain that all the NSTs are fine.

Yep!  14 x 1800v = 25.2 kV +-10%....  So they are about 12.1 kV per leg.
High enough that they should never have to conduct but if something goes
bad they will step in and save the NSTs...

>
>
>>         I wonder if the NSTs were drivin into that odd SLTR region where the
>> current limiting on the NSTs goes away.  I would suggest adding fast blow
>> fuses to the NST inputs (~7A for a 15/30).  The fuses will not blow
>> normally but if the NST starts to draw like 4X the current, they will save
>> save the NSTs.  Even though LTR coils have large caps sizes to prevent
>> overvoltage, if the NST shunts saturate, they become little pole pigs and
>> will charge anything till something blows.
>
>I have a 20A fuse (fast blow, I think) in my variac, which runs the
>NSTs.  I lost two while trying to get the 4 NST system tuned (which is
>one reason why it was removed), but there was no severe "over-current"
>problem with the 3 NST system.  What still puzzles me is that we tripped
>the 15A circuit breaker while this "failure" was going on, yet it hadn't
>tripped with all 4 NSTs hooked up (while blowing two 20A
>fuses), and the motor going full bore (5+A).  I did have 300uF of PFC
>caps hooked up (normally have 240uF for a 3NST system), which I forgot
>to disconnect when I removed the 4th NST, :-o  but I can't imagine that
>would do anything other than force me to tinker with the motor phasing,
>which I had to do anyway.
>
>
>
>>         I suspect you are right about a gap problem.  If the sync gap when 
>out of
>> alighnment at full power it could explain this too.  In other words, if
>> something came loose and the rotor's timing changed things could go very
>> wrong and stuff would start blowing right and left (fuses hopefully).
>> Check the gap timing carefully.  I think you will find it no longer fires
>> where you originally had it set.

I ran models today of this and shockingly they do not predict fuses
blowing.  I think this odd saturation thing is at work here because that is
one thing the models do not predict.  I "think" I have all the other bases
covered...  Apparently, there are conditions where NSTs will draw huge
amounts of current.  I know form the noise and the lights dimming that what
ever is the cause, it is "bad"!

>
>Like I said in my last post, this isn't the first motor failure.  It was
>replaced in a friend's furnace, too.....  The rotor is anchored with a
>1/4-20 set screw.  There is a flat on the motor's shaft that the set
>screw
>presses against.  I tightened the set screw about a week before the demo. 
>The motor cannot turn more than about 180 degrees before the wires stop it
>(and we would have seen the motor move, as we were staring at the rotor the
>whole time).  The screws that mount the motor to its base were tight, and
>had to be loosened to move turn the motor by hand.  In short, I *highly*
>doubt that anything came loose, but I will definately doublecheck next
>weekend.
>
>I have a 10A fuse (slow blow?) in the motor variac, which is about twice
>the operating current.  I use the variac to ramp up the motor, as I have
>routinely blown 10A fuses in the past (draws ~22A startup current), and
>also to try to keep inertial forces due to a hasty startup to a minimum.
>
>Thankfully the street lights allowed us to visualize the motor phasing (the
>Demo was done on a street corner in the middle of DT Kalamazoo!  People
>honked while driving by, even! :-).  This is how I know it was hunting. 
>After cutting NST power, the motor would re-sync.  It's a 1/3HP 1800RPM
>motor, and it has a LOT of extra power for a 10" rotor.
>

I wonder if the motor is fine but the current draw from the coil is so
great that it is affecting the AC power and that is causing the motor to
loose sync.  Check the motor very carefully before saying it is bad.
They're is very little that can go wrong with a motor.  Maybe the coils and
the motor are "talking" to each other on the input AC and it is more a
matter of getting a lower impedance AC line to them both.  Were you using a
long extension cord or any other higher resistance feed line to the coil
and the motor?  If the motor is fine except when the coil is running, the
AC line input to the motor must be seeing some major disturbance.

The motor could simply have a loose connection in it somewhere which is
opening when it gets hot.  It should be easy to find any such problems in
the motor if you open it and have a look around.

Cheers,

	Terry


>
>
>Mark Broker:  Master Procrastinator.